The invention relates to a speed control apparatus for a d.c. motor, and more particularly, to such apparatus which incorporates a switching control.
A conventional speed control apparatus for d.c. motor is illustrated in FIG. 1. Specifically, a d.c. motor 1 defines a branch of a bridge circuit, the remaining branches of which are defined by resistors 2, 3 and 4. The bridge circuit includes a pair of detecting terminals A, B, and a voltage detected thereacross may be compared against a reference voltage 7 by an error detector formed by a combination of a transistor 6 and a resistor 5. The error detector detects a difference therebetween, which may be used to control a feed transistor 8, thus controlling the power supplied to the motor to thereby control the speed thereof. In such an arrangement, the feed transistor 8 operates in its active region, and hence a voltage drop across the emitter and collector represents a Joule loss, causing a reduced efficiency.